What you are experiencing is a hangover. Alcohol dries out the body giving you a headache and making you feel stiff. It's only really long term heavy drinking that causes serious damage, but don't take that as me saying it's completely safe.

What you said about the Nightquil however worries me. Half a bottle is a hell of a lot. Mixing alcohol with any kind of medication desiged to help you sleep is a bad idea. You should only have taken a spoonful if anything at all. The best way to get a good night's sleep and a hangover free day after a good session is keep off caffine while drinking (sorry, no Jagerbombs!), keep active (get dancing) and drink a fudgeload of water and have a pee before going to bed.